The present invention relates to a non-aqueous electrolyte secondary battery.
Nowadays, the manganese dioxide-zinc battery is mainly used as a primary battery for power supply of electronic appliances. Nickel batteries such as the nickel-cadmium battery, the nickel-zinc battery and the nickel-metal hydride battery, as well as the lead acid battery are mainly used as a secondary battery for power supply of electronic appliances.
As the electrolyte for these batteries, there is used an aqueous solution of an alkali such a potassium hydroxide or an aqueous solution of sulfuric acid or the like. The theoretical decomposition voltage of water is 1.23 V. A battery having a voltage higher than 1.23 V can easily undergo decomposition of water and thus can hardly store the electric energy. Thus, a battery having an electromotive force of about 2 V at highest has been put in practical use. Therefore, in order to meet the demand for novel higher performance batteries meeting the development of electronic appliances, a high voltage battery having a voltage of 3 V or higher comprising a non-aqueous electrolyte as an electrolyte has been used. A typical example of such a battery is a lithium battery comprising metallic lithium as a negative active material. Examples of primary lithium battery include manganese dioxide-lithium battery, carbon fluoride-lithium battery, etc. Examples of secondary lithium battery include manganese dioxide-lithium battery, vanadium oxide-lithium battery, etc.
The secondary lithium battery using metallic lithium as the negative active materials is disadvantageous in that metallic lithium causes dendrite deposition, easily causing shortcircuiting that reduces battery life. Further, since metallic lithium has a high reactivity, it is difficult to secure safety. Therefore, a lithium ion battery comprising graphite or carbon instead of metallic lithium as a negative active material and lithium cobalt oxide, lithium nickel oxide or the like as a positive active material has been devised and used as a high energy density battery. However, with the recent expansion of usage, batteries having higher performance, higher energy density and higher safety have been desired.
Thus, a secondary lithium battery having a high energy density comprising metallic lithium as a negative active material is again attracting attention. However, as mentioned above, technical problems of short cycle life can still be hardly overcome, not to mention safety. Thus, such a secondary lithium battery has never been put in practical use.
In other words, when a secondary lithium battery comprising metallic lithium as a negative active material in a negative electrode 25 is repeatedly subjected to charge and discharge cycle, metallic lithium dendrite 21 of metallic lithium is produced and pierces a separator 29 to cause shortcircuiting during charge, as shown in FIG. 3. Further, fine metallic lithium powders 23 which does not participate in charge and discharge is accumulated in the vicinity of the negative electrode 25, lowering the discharge capacity and hence reducing the battery life.
In particular, a non-aqueous electrolyte secondary battery normally comprises as an electrolyte a combustible organic solvent that can cause heat generation and fuming and thus it is required to assure safety sufficiently. Thus, the use of various safety elements and polymer electrolytes which are less reactive with the electrode than the liquid electrolytes have been often attempted. Furthermore, some reports were made on the use of a porous polymer electrolyte and the incorporation of a liquid electrolyte in the pores (PROCEEDINGS of 16th International Electric Vehicle Symposium, 1999, p156). However, the use of these safety elements or polymer electrolytes also are not sufficient for solving the foregoing problems of reduction of life and deterioration of battery safety by the production of metallic lithium dendrite or formation of fine metallic lithium powders.
The foregoing problems with secondary lithium battery comprising metallic lithium can occur also with the case where a lithium alloy or carbon material capable of absorbing and releasing lithium is used. In other words, in the case where the utilization of negative active material during charge or discharge is raised to enhance the energy density of the battery or a high rate or low temperature charge is carried out, metallic lithium dendrite is deposited on the surface of the negative active material, causing the same problem as occurring in the case of metallic lithium negative active material.
Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery having an excellent cycle life performance and an enhanced safety.
The present invention comprises a polymer membrane containing at least one material selected from the group consisting of carbon powder, silicon powder, tin powder and aluminum powder(hereinafter referred to as xe2x80x9ccarbon powder or the likexe2x80x9d), wherein the membrane is provided between a positive electrode and a negative electrode. In accordance with the arrangement of the present invention, the carbon powder or the like, as a lithium-absorbing material, absorbs metallic lithium powders or dendrite which has been produced from the negative electrode due to charge or discharge and takes no part in charge or discharge. Since this lithium-absorbing material is less reactive than metallic lithium powders or dendrite, the resulting battery exhibits an enhanced safety. Further, since the metallic lithium powders or dendrite is absorbed by the carbon powder or the like, the internal shortcircuiting between the positive electrode and the negative electrode can be prevented, drastically improving the charge and discharge cycle life performance.
It is preferred that the polymer membrane containing carbon powder or the like be porous and particularly have a porosity of from 10% to 90%. When the polymer membrane containing carbon powder or the like is made porous, a liquid electrolyte can be retained in the pores. In this arrangement, the flow of liquid electrolyte caused by the volumetric change of the active material during charge or discharge occurs also in the polymer membrane via the liquid electrolyte retained in the pores. Metallic lithium powders or dendrite which has been released from the negative electrode and thus cannot be charged or discharged moves through pores of the polymer membrane on the flow of liquid electrolyte and thus can easily reach the carbon powder or the like.
Alternatively, a separating membrane layer may be further provided between the positive electrode and the polymer membrane.